1. Field of the Invention
The present application relates to a technique to manufacture a field effect transistor at a surface layer part of a non-single-crystal semiconductor thin film, and to single-crystal or polycrystal semiconductor thin film substrate used to manufacture a field effect transistor, and to crystallization method, a crystallization apparatus, a thin film transistor and a display apparatus suitable for manufacture of a display apparatus such as a liquid crystal or an organic EL or an electronic apparatus such as an information processing apparatus in which a field effect transistor are incorporated.
2. Description of the Related Art
A display apparatus such as a liquid crystal display unit is formed at an amorphous semiconductor film formed on a glass substrate. Specifically, as a display mode of a liquid crystal display, there is currently adopted an active matrix mode which switches individual pixels, and an amorphous silicon thin film transistor (a-SiTFT) is mainly used for a pixel switching element. Since information to be processed is digitalized and subjected to an improvement of speed due to expansion of an IT market, high performances is demanded in a display apparatus which displays such information. As means for satisfying this demand, a switching speed is increased by forming a switching transistor of respective pixels on a crystal area, thereby improving the picture quality.
In addition, miniaturization is enabled by having circuit treating data of each pixel built-in.
In the technical development of the liquid crystal display (LCD), the studies are keenly advanced with 1. realization of the high definition, 2. realization of a high numerical aperture, 3. a reduction in weight, 4. a reduction in cost and others being determined as objectives. In order to realize these performances, a technique using a polycrystal semiconductor thin film transistor (Poly-SiTFT) has come to the front. Since the Poly-SiTFT has the higher mobility than that of the a-SiTFT by two figures or more, an element size can be reduced, and a integrated circuit can be formed. Therefore, a drive circuit or an arithmetic operation circuit can be also mounted on the LCD.
A method for manufacturing a polycrystal semiconductor thin film transistor based on an excimer laser crystallization method according to the prior art will now be described with reference to FIGS. 12A to 12D. For example, as shown in FIG. 12A, an underlying protective film (e.g., an SiO2 film, an SiN film, an SiN/SiO2 laminated film or the like) 102 and an amorphous silicon thin film 103 are deposited on a glass substrate 101 as shown in FIG. 12A. Then, as shown in FIG. 12B, an excimer laser (XeCl, KrF or the like) 104 whose beam has been shaped into a square form or an elongated form by an optical system is used to irradiate a surface of the amorphous silicon thin film. Then, the amorphous silicon thin film 103 is converted from an amorphous structure into a polysilicon structure through a melt and solidification process in a very short time of 50 to 100 nano seconds by irradiation and heating of the excimer laser 104. When the entire surface of the amorphous silicon film 103 is scanned and heated by the excimer laser 104 in a direction indicated by an arrow 105, such a polycrystal silicon thin film 106 as shown in FIG. 12C is formed.
The above-described process is called an excimer laser annealing technique (which will be referred to as an ELA method hereinafter). The ELA method is used when manufacturing a high-quality polycrystal thin film on a substrate which is made of a material having a low melting point such as glass. In regard to these points, the detail is described in, e.g., Nikkei Microdevices, separate-volume Flat Panel Display 1999 (Nikkei Business Publications, Inc., 1998, pp. 132-139).
A thin film transistor shown in FIG. 12D is manufactured by using the polycrystal silicon thin film 106 described in FIG. 21C. A gate insulating film 107 such as an SiO2 film is provided on the polycrystal silicon thin film 106 of this TFT by film formation. Further, a source impurity implantation area 108 and a drain impurity implantation area 109 are provided. A gate electrode 110 is provided on the gate insulating film, a protective film 111 is formed, and a source electrode 113 and a drain electrode 114 are formed. The TFT which can control a current between the source and drain by a voltage of the gate electrode is brought to completion by the above-described steps.
However, a grain size of a crystal obtained by this ELA method is approximately 0.1 μm. Therefore, in case of the thin film transistor (TFT) formed in this crystallized area, many crystal grain boundaries exist in a channel area of one thin film transistor. As a result, this transistor has the mobility of 40 to 60 cm2/Vs and an on/off current ratio of approximately 107 and hence it is greatly inferior to an MOS transistor formed at single-crystal Si. Irregularities are generated in characteristics of each thin film transistor due to nonuniformity in the number of crystal grain boundaries and, in particular, there is a problem that this transistor is not suitable for a display apparatus which requires uniform display in one screen.
Furthermore, in order to improve the performances of the TFT, there has been reported a “phase modulation excimer laser crystallization method” which is a technique to single-crystallize polycrystal silicon as a method evolved from the ELA method. In the phase modulation excimer laser crystallization method, laterally grown Si crystal grain r2 whose positions are controlled can be formed by controlling a beam profile B shown in FIG. 2C. As a thesis concerning such a phase modulation excimer laser crystallization method, there is, e.g., The Ninth International Display Workshops (IDW′ 2) Proceedings pp. 263-266. In the phase modulation excimer laser crystallization method, formation of a laterally grown Si crystal grain is facilitated by utilizing a controlled beam profile. In the conventional ELA method, although there is, e.g., Journal of Applied Physics Vol. 88, No. 9, 1 Nov. 2000, pp. 4994-4999 concerning a correlation of a beam profile and a crystallized cell, there is a great difference between these theses since the beam profile is not controlled in the latter.
The TFT characteristics are greatly improved without an adverse affect of the crystal grain boundaries by forming the TFT in a single crystal grain, thereby forming a function element such as processor, a memory, sensor and others. As such a crystallization method, there is, e.g., a crystallization method described in W. Yeh and M. Matsumura, Jpn. Appl. Phys. Vol. 41 (2002) 1909 or a crystallization method described in Japan Society of Applied Physics, the 63rd academic lecture in autumn 2002, preliminary manuscript correction 2, p. 779, 26a-G-2, Masato Hiramatsu and et. al.
The former reference by W. Yeh describes a cap film formed of an SiON/SiO2 film or a cap film formed of an SiO2 film. Phase-modulated laser beam with a fluence of 0.8 J/cm2 is irradiated to an amorphous silicon film through this cap. There is described a method for crystallizing the amorphous silicon film by laterally growing a crystal grain in a direction parallel to the cap film.
Furthermore, the latter reference by Hiramatsu describes irradiation of phase-modulated laser beam which is homogenized to an amorphous silicon film through a cap film formed of an SiO2 film with a substrate being heated. There is described a method by which a melted area of the amorphous silicon film can be subjected to crystal growth in the lateral direction.
However, when the silicon film is crystallized by using the conventional phase modulation excimer laser crystallization method, the following problem occurs. Laterally grown Si crystal grains r2 as well as polycrystal grains r3 on the outer thereof are generated as shown in FIG. 2D, a fine crystal grain r4 is produced at the center, and crystal grain breaking areas r5 may be further generated. That is because a light intensity distribution is not optimized.
Actually observing a structure crystallized by the phase modulation excimer laser crystallization method, single-crystallized areas r2 with a large grain size are generated, but other undesirable cells r4 and r5 are also produced.
Moreover, the method described in the reference by W. Yeh can obtain a crystal grain with a large grain size which is not less than a crystal grain size 10 μm. However, a fine crystal grain with a small grain size may be generated in the vicinity of a crystal grain grown to have a large grain size in some cases, and there is a demand to relatively evenly (i.e., densely) form crystal grains with a large grain size all together as an entire film structure.
Additionally, in the methods described in the reference by W. Yeh and the reference by Hiramatsu, there is a demand for low-temperature or ordinary-temperature processing with respect to a temperature of a substrate in order to increase a grain size of crystal grains. For example, in a conventional crystallization apparatus 300 shown in FIG. 31, laser beam 250 is irradiated while heating a substrate 5 in a high-temperature area by using a heater 301 included in a mount base 206. The heater 301 receives a power from a power supply 302 which is controlled by a controller 303, and has a capability to heat the substrate 5 to a temperature area of 300 to 750° C.
The substrate heating temperature may exceeds, e.g., 500° C. in some cases. Then, general-purpose glass (e.g., soda glass) or plastic is apt to be transformed or deformed due to heating, and the low-temperature processing is a prerequisite in order to adopt such general-purpose glass for a substrate in a liquid crystal display (LCD). Further, in a large-screen LCD, there is a tendency to reduce a plate thickness of a substrate since there is a strong demand to reduce a weight thereof, and deformation is apt to occur due to heating. Therefore, the low-temperature processing is a prerequisite in order to assure the flatness of a thin substrate.
It is an object of the present invention to provide a crystallization method and apparatus, a thin film transistor and display apparatus which can design pulse laser beam (“laser beam” described below means pulse laser beam) having a light intensity and a distribution optimized on an incident surface of a substrate, form a desired crystallized structure while suppressing occurrence of other undesirable structure areas and satisfy a demand for low-temperature processing.